What would actually happen to Earth if a large asteroid hit the ocean instead of land?

An asteroid impact in the ocean would be catastrophic in ways that differ significantly from a land impact, though both would be devastating. The immediate effect would depend heavily on the size of the impactor. A relatively modest asteroid, say 200 to 500 meters in diameter, striking the ocean would vaporize an enormous volume of seawater almost instantly. This vaporization would create a massive steam explosion and a transient cavity in the ocean floor that could be several kilometers wide and deep. The surrounding water would rush back in violently, creating a series of enormous waves radiating outward in all directions. These would not be ordinary tsunamis caused by seafloor displacement. They would be impact-generated mega-tsunamis, potentially hundreds of meters tall near the impact site, though they would diminish in height as they spread across the ocean basin. The tsunami threat is actually one of the most dangerous aspects of an ocean impact compared to a land strike. Coastal populations around entire ocean basins could be inundated within hours. A large impact in the middle of the Atlantic, for example, could send devastating waves crashing into the eastern seaboard of North America and the western coasts of Europe and Africa simultaneously. Cities like New York, Lisbon, and Lagos could face walls of water tens of meters high. The flooding would penetrate far inland in low-lying areas, and the death toll from the tsunamis alone could dwarf almost any other natural disaster in recorded history. The atmosphere would take a severe beating as well. The vaporized seawater would inject an almost incomprehensible amount of water vapor and steam into the upper atmosphere. This would initially cause intense greenhouse warming because water vapor is a potent greenhouse gas. However, the impact would also throw up enormous quantities of pulverized rock from the ocean floor, along with soot and debris, which would block sunlight and potentially trigger a period of impact winter. The interplay between these effects is complex, but climate disruption would be severe and prolonged. Acid rain would be another consequence, as the heat of impact would cause atmospheric nitrogen and oxygen to combine into nitrogen oxides, which then react with water vapor to form nitric acid that falls as rain, damaging ecosystems and agriculture across wide regions. For a truly large impactor, something in the range of a kilometer or more in diameter, the ocean impact would essentially be globally catastrophic regardless of where it hit. The energy released would be so enormous that the distinction between ocean and land becomes almost irrelevant at the planetary scale. The Chicxulub impactor that killed the dinosaurs was roughly 10 to 15 kilometers wide and struck what is now the Gulf of Mexico, a shallow sea at the time. That event triggered wildfires across continents, a years-long impact winter, massive acid rain, and the collapse of food chains worldwide. An ocean impact of similar scale would produce comparable results, with the added dimension of even more water vapor injection into the atmosphere and potentially more intense initial tsunami activity. One aspect that sometimes surprises people is that an ocean impact does not necessarily mean less damage to land. In some respects it can be worse for coastal regions because of the tsunami amplification effect. A land impact in a remote area might devastate a large region through blast, fire, and ejecta, but the destruction would be more localized in the immediate aftermath. An ocean impact spreads its destructive energy across the water and delivers it to coastlines where a large fraction of the world's population lives. The vaporized seawater also means more material injected into the atmosphere compared to a dry land impact, potentially intensifying some of the longer-term climate effects. Scientists studying impact scenarios generally consider ocean impacts to be at least as dangerous as land impacts of equivalent size, and for mid-sized impactors the tsunami threat makes coastal ocean impacts particularly lethal for human civilization. The good news, if there is any, is that the deep ocean floor would absorb some of the energy that would otherwise go into launching ejecta into the upper atmosphere, slightly reducing one of the mechanisms that contributes to global impact winter.